Is it possible to reverse a "paid invoice" back to an open unpaid invoice

I can help achieve this task so this invoice will back to its unpaid status, @mike-james.

You can delete the payment you applied to this transaction. By this, the program automatically changes the status of the invoice to open or unpaid. 

Here are the steps to do it:

1. Select the Sales or Invoicing menu, then go to the Invoices tab.
2. Locate the invoice and click the View/Edit option in the dropdown.
   
3. Select the 1 payment link at the upper right hand above the PAID payment status.
4. Click the Date link.
   
5. On the Receive Payment window, press More at the bottom.
6. Choose Delete, then click Yes.
   

You can refer to this reference to receive and record invoice payments in QuickBooks Online. This provides ways to handle sales transactions to make sure QuickBooks processes the payment and categorizes it into the correct account.

If you have other concerns besides reversing a payment, just add a comment below. I’ll be here to help you out. Have a good one!